Have been very pleased with DVR, but as the reviews all seem to say, the fan is quite noisy in a domestic environment. Has anyone replaced the fan with a quieter type?

It depends on whether you are a "tinkerer", and can make some mods.

If you notice the fan is installed in a way that you cannot bolt on a traditional size computer fan. If you find a fan that will fit, and it is quiet, but if you are still concerned about how much air it draws then perhaps add some heatsinks on to the IC chips to help pull the heat out.

If you get a really good fan, but you are unsure of how much amperate it pulls then perhaps you can build/install your own fans power supply to avoid sucking you AVTech power suppy to death.